Quotas in PowerMail work somewhat different from other mail solutions. A message with a single recipient is quite standard: all messages on all backends are accounted for, whereby each message is calculated to use 4kb minimum, which is actually in line with the amount of disk space occupied.
If there is room for the new message, it is stored.
However, if a message has multiple recipients, things are a bit different. As PowerMail uses hardlinks to deliver messages space efficiently, a message is accepted if at least *one* of the recipients has enough room in his mailbox.
This has the additional benefit of making sure that 'messages to everybody' never bounce. Furthermore, users perceive more diskspace than they actually use.